Hi, mbasan,

Thanks for the interest to our paper. Following are our response to your questions:


1. In the images you show in your paper there is a large accumulation of micro beads around the oblate tumor tissue. However for the spherical tumor tissue this accumulation appears to be missing. Assuming an initially homogeneous distribution of beads, we would also expect a large accumulation of beads in the vicinity of the tumor in the spherical case as well. Where have the beads that were in the volume, which is now filled by the tumor gone? The spherical tumor has stopped growing and it is also exerting a force on the gel?

There is actually significant accumulation of beads around spherical tumors as well, but this effect doesn't appear obvious in 2D image slices at the spheroids' equator (Fig. 1A). In projections through more depth the change in bead density is more obvious. Note that there was a trade-off in choosing the bead density—higher densities lead to problems with overlap and saturation in the images From 3D image stacks of the spheroid surface and the surrounding beads above the spheroid's equator (i.e., when a much bigger sampling volume is used), the bead accumulation can be quantified (Fig. 1B) and there is a consistent, significant increase in bead density within 50 um from the spheroid's surface. So yes, spherical tumors do exert mechanical stress on the gel. See Fig 1C for the correlation between the size of spherical tumors and the resulting strain in the gel.


2. In the framework of our mansucript ( http://arxiv.org/PS_cache... ), we would expect that when you inhibit apoptosis the tumor grows to a larger size and deforms the gel more. Do you observe this in your experiments?

We would also expect spheroid size to increase if apoptosis is inhibited, but haven’t yet performed a focused analysis of the growth curves in this case.


3. When most of the cells in the tissue underwent apoptosis, did you observe a relaxation of the stress in the gel?

We think that cancer cell proliferation, mechanical stress accumulation and stress-induced cell apoptosis finally reach an equilibrium where the stress is high enough to cause the amount of apoptosis to balance cell proliferation so that the spheroid’s size stops increasing. Gel relaxation will likely happen if you induce extra apoptosis with reagents such as Taxol or Doxorubicin, but we haven't tried that.